Many vertebrates, including man, lack the ability to manufacture a number of amino acids and therefore require these amino acids preformed in the diet. These are called essential amino acids. Human food and animal feed, derived from many grains, are deficient in essential amino acids, such as lysine, the sulfur amino acids methionine and cysteine, threonine and tryptophan. For example, in corn (Zea mays L.) lysine is the most limiting amino acid for the dietary requirements of many animals. Soybean (Glycine max L.) meal is used as an additive to corn-based animal feeds primarily as a lysine supplement. Thus, an increase in the lysine content of either corn or soybean would reduce or eliminate the need to supplement mixed grain feeds with lysine produced via fermentation of microbes. Furthermore, in corn the sulfur amino acids are the third most limiting amino acids, after lysine and tryptophan, for the dietary requirements of many animals. The use of soybean meal, which is rich in lysine and tryptophan, to supplement corn in animal feed is limited by the low sulfur amino acid content of the legume. Thus, an increase in the sulfur amino acid content of either corn or soybean would improve the nutritional quality of the mixtures and reduce the need for further supplementation through addition of more expensive methionine.
Lysine, threonine, methionine, cysteine and isoleucine are amino acids derived from aspartate. Regulation of the biosynthesis of each member of this family is interconnected (see FIG. 1). One approach to increasing the nutritional quality of human foods and animal feed is to increase the production and accumulation of specific free amino acids via genetic engineering of this biosynthetic pathway. Alteration of the activity of enzymes in this pathway could lead to altered levels of lysine, threonine, methionine, cysteine and isoleucine. However, few of the genes encoding enzymes that regulate this pathway in plants, especially corn, soybeans and wheat, are available.
The organization of the pathway leading to biosynthesis of lysine, threonine, methionine, cysteine and isoleucine indicates that over-expression or reduction of expression of genes encoding, inter alia, threonine synthase, dihydrodipicolinate reductase, diaminopimelate epimerase, threonine deaminase and S-adenosylmethionine synthetase in corn, soybean, wheat and other crop plants could be used to alter levels of these amino acids in human food and animal feed. Accordingly, availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate development of nutritionally improved crop plants.